1. Technical Field
This invention relates to testing techniques and, more particularly, to a method and apparatus for testing infrared detectors.
2. Discussion
Infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electromagnetic radiation having wavelengths in the range of about 1-15 micrometers. Mercury-cadmium-telluride or indium antimonide detector arrays are often utilized and they are generally mounted in a cryogenically cooled dewar that cools the arrays to a temperature of around 77.degree. K. where they are most sensitive. Silicon or germanium detectors often are used at lower temperatures, 15.degree. K. to 25.degree. K., or even colder. Infrared detectors of this type are well known in the art and need not be discussed in detail herein since the present application is directed to the testing of these detectors and not their manufacture.
FIG. 1 illustrates a prior art method of testing an infrared detector 10 having a detector array 12 mounted within a dewar 14. A blackbody source of infrared radiation is generally designated by the reference numeral 16. The blackbody 16 is typically heated to around 500.degree. K. so that it irradiates radiation have a wavelength in the infrared spectrum of interest, here, some portion of the band of 1-15 micrometers. The radiation from source 16 is typically concentrated by a water cooled aperture 18 having a given aperture size; this may be water-cooled to maintain the desired near-ambient temperature needed for it to correctly restrict the subtense of the hot radiator (the radiation source). Details of conventional blackbody designs are disclosed in R. D. Hudson, Infrared System Engineering (John Wiley & Sons, 1969), which is hereby incorporated by reference. The resulting beam 20 of radiation is directed to the detector array 12. The beam 20 is periodically blocked by a spoked wheel or other shutter-type chopper 22 which modulates the beam 20 by alternatively blocking and then passing the beam from the source 16 to the detector array 12. This modulated beam is used as a test pattern where the output of the detector array 12 is measured and compared against expectant results. Usually, the fundamental sinusoidal component of the harmonically distorted detector waveform is measured, then adjusted by a waveform factor to derive the desired peak waveform amplitude expressing detector response to the source. Then, trim components are utilized to optimize the response of the array 12.
While this standard test procedure has met the test of time, it does have some drawbacks. In general, it would be more advantageous to be able to test the detectors under conditions which more closely represent the conditions in which the detector will actually be used. However, the necessity of heating the blackbody source to 500.degree. K. does not lend itself to accomplishing this goal since the detector generally will be used at room temperature, about 300.degree. K. Accordingly, the test data may not simulate actual operating conditions. In addition, the prior art technique utilizes a relatively small aperture size (typically on the order of 0.25 to 1.0 inch) for restricting the beam from the high temperature blackbody source. This is necessary to avoid unwanted radiative heating of objects near the source, and to avoid undue radiative cooling of the source due to excessive escape of heat. That is, the aperture contains the heat within the blackbody. The aperture size serves as an adjustment of signal level, inasmuch as temperatures usually are standardized, e.g., at 500.degree. K. The small aperture size results in a narrow cone of test radiation (about f/10-f/20) which is a relatively poor match to the broad cone (e.g., f/5.0 to f/1.0) usually provided by the system optics of typical infrared detectors. As a result, further errors via angular sensitivities of the filters and detectors can be introduced.
When the detector array has a wide field of view it is typically necessary to reposition the detectors during the test, again due to the relatively narrow cone of the test radiation. For best sensitivity, so-called background-limited infrared systems often are designed so that their optics produce a small exit pupil that is matched to detector cold shield, such that only rays contributing to imaging reach the detector; rays adding nonimaging or background flux are blocked. This can result in widely differing angles of incidence at different detector elements. This geometry is poorly simulated by the usual test set, which has only a narrow beam of radiation.
Source calibration is typically complicated by the enclosed nature and high operating temperature of the blackbody. Inserting thermocouples for calibration can damage the surfaces, while visual inspection, cleaning and refinishing of the surfaces can be difficult.
It is the object of the present invention to solve one or more of these problems.